PURPOSE: To analyze the clinical and radiological result of diaphyseal fractures of the forearm both bones treated by plate fixation and plate fixation with intramedullary nailing. MATERIALS AND METHODS: We reviewed 52 cases of diaphyseal fractures of the forearm both bones in adults that were treated and the follow-up period was 1 year above. The first group(I), 25 cases(48.1%) were treated with plate fixation in radius and ulna, the second group(II), 27 cases(51.9%) were treated with plate fixation in radius and threaded Steinman pin fixation in ulna. we analyzed the results by average union time and functional result according to Anderson's criteria. RESULTS: The mean duration of union was in the first group, 12.3 weeks in the second group, 13.2 weeks. By Anderson's criteria, in the first group, 21 cases(84%) and in the second group, 22 cases(81.5%) had a good result. As complications in the first group, non-union 2 case, angulation deformity 1 case, rotational deformity 1 case and in the second group, non-union 4 cases, angulation deformity 2 cases. CONCLUSION: We considered that satisfactory results can be obtained by rigid internal fixation with plates in radius & ulna and early mobilization in fractures of forearm both bones in adults and according to the type of fracture, Fixation with plate in radius and threaded Steinman pin in ulna was one of the proper methods.